Hydraulic fracturing is a well stimulation technique designed to increase productivity of an oil or gas well by creating highly conductive fractures in the producing geologic formation around the well. The process involves injecting a fluid at a high flow rate and high pressure to rupture the formation and create cracks in the rock and pumping into these cracks a fluid containing a particulate material (propping agent or proppant) to maintain the cracks or fractures open by resisting the forces which tend to close them.
The list of materials used as proppants is rather long and includes sand, metal particles, sintered bauxite, sintered alumina, and refractories such as mullite and glass beads. Generally speaking, lower specific gravity proppants can be carried further into fractures than those of higher specific gravity. Lower specific gravity proppants permit decreases in pumping pressure during proppant placement which reduces bottom hole pressure. In addition, lower proppant density allows for the use of less expensive fracturing fluids.
Gravel packing is a technique used to prevent formation silt from restricting oil or gas flow into the well bore to improve the flow of hydrocarbon into the producing formation or from the formation into the well bore. This can be done by introducing a coarse packing into such wells. It is typically used in deviated wells wherein the well casing is perforated by controlled explosive charge in selected sections. The gravel pack keeps formation sand from flowing back into the perforations created by the explosive charge and yet is porous enough to allow free flow of the oil or gas through the well.
Gravel pack material is desirably strong, low in density, and resistant to solvents and acids. Specifications for gravel pack are given in RP56 and RP58 of the American Petroleum Institute (API). Some ceramics tested for use in gravel packing have degraded over time in the environmental conditions likely to be experienced in deep wells by a phenomenon known as stress corrosion. Many organic coatings which could be applied to ceramics to protect them from down hole conditions are adversely affected by solvents present in crude oil. In addition, many organic (plastic) materials tend to form discontinuous coatings.
Sand is a relatively good gravel pack material except for its high specific gravity of about 2.65. Bauxite and clay compositions have poor mud acid resistance in addition to high specific gravity. Low specific gravity spheres, such as perlite lack mud acid resistance and sufficient strength (crush resistance). Polypropylene and other polymeric spheres tend to deform and compact under pressure, especially at temperatures likely to be found in oil wells, 66.degree. C. or higher.
The ceramic spheroids of U.S. Pat. No. 4,632,876 have been considered for use as a gravel pack. However, the resistance of those embodiments tested to acids such as hydrofluoric and hydrochloric acids is considered too low.